What is an electric heater
An electric heater is an internationally popular electric heating device. It is used for heating, heat preservation and heating of flowing liquid and gaseous media. When the heating medium passes through the heating chamber of the electric heater under the action of pressure, the principle of fluid thermodynamics is used to uniformly take away the huge heat generated by the electric heating element, so that the temperature of the heated medium can meet the user’s technological requirements.
Electric heater power calculation
w=p×t×η can get p=w÷t÷η, here is the formula to find the power.
First we find w=cm(t2-t1)=4.2×10×1000×(50-20)=1260000J. The specific heat capacity of water is 4.2×103 J/(kg°C), the mass of water = density multiplied by volume = 1.0X10^3kg/m^3×10m3
p=1260000÷1200÷0.8=1.3125 kW.
As a physical quantity that expresses the speed of current doing work, the magnitude of the power of an electrical appliance is numerically equal to the electrical energy it consumes in 1 second. If the electric energy “W” (SI unit is J) consumed in such a long time of “t” (SI unit is s), then the electric power of this electrical appliance is P=W/t (definition formula) The electric power is equal to the two ends of the conductor The product of voltage and current through a conductor.
(P=U·I). For purely resistive circuits, the formulas P=I^2*R and P=U^2 /R can also be used to calculate the electrical power.
The use of electric heaters
According to the analysis of the “2013-2017 China Electric Heater Industry Development Prospect and Transformation and Upgrading Analysis Report”, the use of electric heaters is mainly in the following five aspects:
1. Heat treatment: local or overall quenching, annealing, tempering, heat penetration of various metals;
2. Hot forming: whole piece forging, partial forging, hot heading , hot rolling;
3. Welding: brazing of various metal products, welding of various tool blades, saw blade serrations, steel pipe, copper pipe welding, welding of the same and dissimilar metals;
4. Metal smelting: (vacuum) smelting, casting and evaporative coating of gold, silver, copper, iron, aluminum and other metals;
5. Other applications of high-frequency heating machine: semiconductor single crystal growth , heat fit, bottle heat sealing, toothpaste skin heat sealing, powder coating, metal implanted plastic.
Sixth, the heater is the most popular electric heating equipment in today’s society, it not only has high quality, long life. It is for flowing liquids.
The heating method of electric heaters is resistance heating
The Joule effect of electric current is used to convert electrical energy into thermal energy to heat objects. Usually divided into direct resistance heating and indirect resistance heating. The power supply voltage of the former is directly applied to the object to be heated, and when there is current flowing, the object to be heated (such as an electric heating iron) will heat up. Objects that can be directly resistively heated must be conductors with high resistivity. Since the heat is generated from the heated object itself, it belongs to internal heating, and the thermal efficiency is very high. Indirect resistance heating requires special alloy materials or non-metallic materials to make heating elements, which generate heat energy and transmit it to the heated object through radiation, convection and conduction. Since the object to be heated and the heating element are divided into two parts, the types of objects to be heated are generally not limited, and the operation is simple.
The materials used in the heating element of indirect resistance heating generally require high resistivity, small temperature coefficient of resistance, small deformation at high temperature and not easy to be embrittled. Commonly used are metal materials such as iron-aluminum alloy, nickel-chromium alloy, and non-metallic materials such as silicon carbide and molybdenum disilicide. The maximum working temperature of metal heating elements can reach 1000-1500℃ according to the type of material; the maximum working temperature of non-metal heating elements can reach 1500-1700℃. The latter is easy to install and can be replaced by a hot furnace, but it needs a voltage regulator when working, and its life is shorter than that of alloy heating elements. It is generally used in high temperature furnaces, places where the temperature exceeds the maximum working temperature of metal heating elements and some special occasions.
Induction heating
The heating effect of induced current (eddy current) generated by the conductor in the alternating electromagnetic field makes the conductor itself heat up. According to different heating process requirements, the frequency of AC power supply used in induction heating includes power frequency (50-60 Hz), intermediate frequency (60-10000 Hz) and high frequency (higher than 10000 Hz). The power frequency power supply is the AC power supply usually used in industry. The power frequency of most countries in the world is 50 Hz. The voltage applied to the induction device by the power frequency power supply for induction heating must be adjustable. According to the power of the heating equipment and the capacity of the power supply network, a high-voltage power supply (6-10 kV) can be used to supply power through a transformer; the heating equipment can also be directly connected to a 380-volt low-voltage power grid.
The medium frequency power supply has used medium frequency generator sets for a long time. It consists of an intermediate frequency generator and a driving asynchronous motor. The output power of such units is generally in the range of 50 to 1000 kilowatts. With the development of power electronic technology, thyristor inverter intermediate frequency power supply has been used. This intermediate frequency power supply uses a thyristor to first convert the power frequency alternating current into direct current, and then convert the direct current into alternating current of the required frequency. Due to the small size, light weight, no noise, reliable operation, etc. of this frequency conversion equipment, it has gradually replaced the intermediate frequency generator set.
The high-frequency power supply usually uses a transformer to raise the three-phase 380 volt voltage to a high voltage of about 20,000 volts, and then uses a thyristor or high-voltage silicon rectifier to rectify the power frequency alternating current into direct current. , and then use the electronic oscillator tube to convert the direct current into high frequency, high voltage alternating current. The output power of high-frequency power supply equipment ranges from tens of kilowatts to hundreds of kilowatts.
The object to be heated by induction must be a conductor. When high-frequency alternating current passes through the conductor, the conductor produces a skin effect, that is, the current density on the surface of the conductor is large, and the current density in the center of the conductor is small.
Induction heating can uniformly heat the whole object and the surface layer; it can smelt metal; in high frequency, change the shape of the heating coil (also known as the inductor), and can also perform arbitrary local heating.
Arc Heating
Uses the high temperature produced by an electric arc to heat objects. Arc is the phenomenon of gas discharge between two electrodes. The voltage of the arc is not high but the current is very large, and its strong current is maintained by a large number of ions evaporated on the electrode, so the arc is easily affected by the surrounding magnetic field. When an arc is formed between the electrodes, the temperature of the arc column can reach 3000-6000K, which is suitable for high-temperature smelting of metals.
Arc heating can be divided into direct and indirect arc heating. The arc current of direct arc heating directly passes through the object to be heated, and the object to be heated must be an electrode or medium of the arc. The arc current of indirect arc heating does not pass through the heated object, and is mainly heated by the heat radiated by the arc. The characteristics of arc heating are: high arc temperature and concentrated energy. However, the noise of the arc is large, and its volt-ampere characteristics are negative resistance characteristics (drop characteristics). In order to maintain the stability of the arc when the arc is heated, the instantaneous value of the circuit voltage is greater than the arc starting voltage value when the arc current instantaneously crosses zero, and in order to limit the short-circuit current, a resistor of a certain value must be connected in series in the power circuit.
Electron beam heating
The surface of an object is heated by bombarding the surface of an object with electrons moving at high speed under the action of an electric field. The main component for electron beam heating is the electron beam generator, also known as the electron gun. The electron gun is mainly composed of cathode, condenser, anode, electromagnetic lens and deflection coil. The anode is grounded, the cathode is connected to the negative high position, the focused beam is usually at the same potential as the cathode, and an accelerating electric field is formed between the cathode and the anode. The electrons emitted by the cathode are accelerated to a very high speed under the action of the accelerating electric field, focused by the electromagnetic lens, and then controlled by the deflection coil, so that the electron beam is directed towards the heated object in a certain direction.
The advantages of electron beam heating are: ① Controlling the current value Ie of the electron beam can change the heating power easily and quickly; ② Using the electromagnetic lens, the heated part can be changed freely or the electron beam can be adjusted freely The area of the bombarded part; ③ The power density can be increased, so that the material at the bombarded point evaporates in an instant.
Infrared heating
Using infrared radiation to radiate the object, after the object absorbs the infrared light, it converts the radiant energy into heat energy and is heated.
Infrared is an electromagnetic wave. In the solar spectrum, outside the red end of visible light, it is an invisible radiant energy. In the electromagnetic spectrum, the wavelength range of infrared rays is between 0.75 and 1000 microns, and the frequency range is between 3 × 10 and 4 × 10 Hz. In industrial applications, the infrared spectrum is often divided into several bands: 0.75-3.0 microns are near-infrared regions; 3.0-6.0 microns are mid-infrared regions; 6.0-15.0 microns are far-infrared regions; 15.0-1000 microns are extremely far-infrared regions Area. Different objects have different abilities to absorb infrared rays, and even the same object has different abilities to absorb infrared rays of different wavelengths. Therefore, in the application of infrared heating, a suitable infrared radiation source must be selected according to the type of the heated object, so that the radiation energy is concentrated in the absorption wavelength range of the heated object, so as to obtain a good heating effect.
Electric infrared heating is actually a special form of resistance heating, that is, a radiation source is made of materials such as tungsten, iron-nickel or nickel-chromium alloy as a radiator. When energized, it generates heat radiation due to its resistance heating. Commonly used electric infrared heating radiation sources are lamp type (reflection type), tube type (quartz tube type) and plate type (planar type). The lamp type is an infrared bulb with a tungsten filament as the radiator, and the tungsten filament is sealed in a glass shell filled with inert gas, just like an ordinary lighting bulb. After the radiator is energized, it generates heat (the temperature is lower than that of general lighting bulbs), thereby emitting a large amount of infrared rays with a wavelength of about 1.2 microns. If a reflective layer is coated on the inner wall of the glass shell, the infrared rays can be concentrated and radiated in one direction, so the lamp-type infrared radiation source is also called a reflective infrared radiator. The tube of the tube-type infrared radiation source is made of quartz glass with a tungsten wire in the middle, so it is also called a quartz tube-type infrared radiator. The wavelength of infrared light emitted by lamp type and tube type is in the range of 0.7 to 3 microns, and the working temperature is relatively low. The radiation surface of the plate-type infrared radiation source is a flat surface, which is composed of a flat resistance plate. The front of the resistance plate is coated with a material with a large reflection coefficient, and the reverse side is coated with a material with a small reflection coefficient, so most of the heat energy is radiated from the front. The working temperature of the plate type can reach more than 1000 ℃, and it can be used for annealing of steel materials and welds of large-diameter pipes and containers.
Because infrared rays have strong penetrating ability, they are easily absorbed by objects, and once absorbed by objects, they are immediately converted into heat energy; the energy loss before and after infrared heating is small, the temperature is easy to control, and the heating quality is high. Therefore, Infrared heating applications are developing rapidly.
Dielectric heating
The use of high frequency electric fields to heat insulating materials. The main heating object is the dielectric. When the dielectric is placed in an alternating electric field, it will be repeatedly polarized (under the action of the electric field, the surface or interior of the dielectric will have equal and opposite charges), thereby converting the electric energy in the electric field into heat energy.
The frequency of the electric field used for dielectric heating is very high. In the medium, short-wave and ultra-short-wave bands, the frequency is from several hundred kilohertz to 300 MHz, which is called high-frequency medium heating. If it is higher than 300 MHz and reaches the microwave band, it is called microwave medium heating. Usually high-frequency dielectric heating is carried out in the electric field between the two polar plates; while microwave dielectric heating is carried out in a waveguide, a resonant cavity or under the irradiation of the radiation field of a microwave antenna.
When the dielectric is heated in a high-frequency electric field, the electric power absorbed in its unit volume is P=0.566fEεrtgδ×10(W/cm)
If it is expressed as heat, it is:
H=1.33fEεrtgδ×10 (cal/sec·cm)
where f is the frequency of the high-frequency electric field, εr is the relative permittivity of the dielectric, and δ is the dielectric loss angle, and E is the electric field strength. It can be seen from the formula that the electric power absorbed by the dielectric from the high-frequency electric field is proportional to the square of the electric field strength E, the frequency f of the electric field, and the loss angle δ of the dielectric. E and f are determined by the applied electric field, while εr depends on the properties of the dielectric itself. Therefore, the objects of medium heating are mainly substances with large medium loss.
Dielectric heating Since the heat is generated inside the dielectric (the object to be heated), the heating speed is fast, the thermal efficiency is high, and the heating is uniform compared to other external heating.
Media heating is used in industry to heat thermal gels and dry grain, paper, wood, and other fibrous materials; it can also preheat plastics before molding, as well as rubber vulcanization and wood, plastic etc. bonding. By choosing the appropriate electric field frequency and device, it is possible to heat only the adhesive when heating the plywood, without affecting the plywood itself. For homogeneous materials, bulk heating is possible.
